埃塞俄比亚西部布雷地区类埃达克岩年代学、地球化学及对区域构造演化的指示

Geochronology,geochemistry and its implication for regional tectonic evolution of adakite-like rockinthe Bure area,western Ethiopia

长期以来,关于埃塞俄比亚西部布雷地区的岩浆活动及构造演化的研究相对较少。本文选择埃塞俄比亚西部布雷地区的花岗岩,通过开展锆石U-Pb定年、全岩地球化学等工作,结果显示:布雷花岗岩LA-ICP-MS锆石U-Pb年龄为733.8±8.1 Ma,属新元古代岩浆作用的产物。布雷花岗岩具有高的SiO2(72.26%~72.78%)和Al2O3(14.91%~15.82%)、低MgO和相对低的Na2O/K2O比值(1.13~1.38),高Sr(310×10^(-6)~401×10^(-6))、Sr/Y(64.9~113.6)、La/Yb(25.7~51.6)和低的HREE、Y(2.71×10^(-6)~4.78×10^(-6))和Yb(0.20×10^(-6)~0.31×10^(-6))和(La/Yb)N(18.4~37.0;<50),具有类埃达克岩的特征,推测其可能为加厚下地壳部分熔融形成。这主要是由于埃塞俄比亚西部布雷地区在750~710 Ma处于东非造山运动的主碰撞阶段,弧陆(陆陆)碰撞使得下地壳厚度不断增加,进而发生部分熔融形成。

For a long time,there has been a lack of accurateconstraint on the magmatic activity and tectonic evolution in the western Ethiopia.Thus,we selected the granite in the Bure area,western Ethiopia,and carried out zircon U-Pb dating and whole-rock geochemicalanalysis.The results show that the LA-ICP-MS zircon U-Pb age of the Bure granite is 733.8±8.1 Ma,which is the product of Neoproterozoic magmatic activity.The high SiO_(2)(72.26%~72.78%)and Al_(2)O_(3)(14.91%~15.82%),low MgO and relatively low Na_(2)O/K_(2)O ratio(1.13~1.38),high Sr(310×10^(-6)~401×10^(-6)),Sr/Y(64.9~113.6),La/Yb(25.7~51.6)and low HREE,Y(2.71×10^(-6)~4.78×10^(-6))and Yb(0.20×10^(-6)~0.31×10^(-6))and(La/Yb)N(18.4~37.0;<50)of Bure granite show the characteristics of adakite,which is mainly formed by the partial melting of the thickened lower crust.TheBure area of western Ethiopia was in the main collision stage of East African orogeny at 750~710 Ma,resulting inthe partial melting of the thick newborn lower crust by the arc-continent(continent-continent)collision.